1. Field of the Invention
The present invention relates generally to a bicycle transmission mechanism, and more particularly, to a multi-speed transmission employing a sprocket gear of the bicycle transmission mechanism that facilitates movement of the chain from one sprocket to an adjacent sprocket during derailing.
2. Description of the Background Art
The bicycle gear-shifting mechanism is generally composed of two sprocket sets and a sprocket chain interconnecting the two sprocket sets for transferring motion from one sprocket to another. The sprocket sets are provided respectively with gears having various numbers of teeth. The gear-changing process of the bicycle is attained by an action of pulling the transmission cable so as to actuate the sprocket chain to move from one sprocket to another.
A derailleur mechanism allows a rider to selectively move the chain to a different one of the sprockets in each set of sprockets. This enables the rider to select a gear ratio that is most appropriate for the current riding conditions.
There have been many attempts to improve upon the derailing of the chain from one sprocket to an adjacent sprocket. Presently, there exist many types of transmissions that employ a sprocket gear and a derailer gear for selectively moving a chain from one sprocket to another adjacent sprocket so as to change the speed of the bicycle.
Basically, derailers function to exert a force on the chain in a location proximate to the sprocket gear so as to force the chain into alignment with the adjacent sprocket. During derailing from one sprocket to another sprocket, the chain is forcibly urged off the teeth of the one sprocket and onto the teeth of the adjacent sprocket. Upon a full revolution, the chain is thus fully derailed from the one sprocket and is fully engaged around the adjacent sprocket, thereby completing the derailing to the adjacent sprocket.
Sprocket gear and derailers have been universally accepted throughout the bicycle industry. However, it has also been widely known that the chain may slip during derailing should the teeth of the adjacent sprocket not fully engage the chain as the chain is forcibly moved off the one sprocket onto the adjacent sprocket. Slippage during shifting is highly undesirable as it causes the bicyclist to lose cadence. Moreover, chain slippage during a power stroke may cause the bicyclist to lose control of the bicycle and may even result in an accident.
A disadvantage of currently available derailleur systems is that they do not always shift smoothly under load, for example, when a rider is riding a bicycle up a hill. A further disadvantage of some derailleur type multi-speed transmissions is that a significant amount of force is generally required to shift the chain between adjacent sprockets in one of the sets of sprockets. Therefore, if it is desired to automate the shifting function by controlling shifting with a computer-controlled actuator; for example, the actuator must be capable of supplying the necessary force. Consequently, an expensive and typically power hungry actuator is required. The result is that electronically controlled automatic derailleur type transmissions have not been widely accepted.
A major problem of the prior art is that the conventional variable speed bicycle uses the conventional sprocket of the single speed bicycle, without taking into consideration the design of the chain shifting process. In other words, when the gear changing is under way, the sprocket chain engages the other sprocket in a random manner. As a result, the chance of success of gear changing of the conventional variable speed bicycle is relatively low. In the meantime, the speed changing process is slow while the operation feel is less smooth. In view of these drawbacks, various improvements in the sprocket structure of the variable speed bicycle have been made in recent years in the hope that the engagement of the sprocket chain and the sprocket takes place successfully with precision when the bicycle gear changing is under way.
U.S. Pat. No. 5,192,248 entitled “Multi-stage Sprocket Gear for Bicycle” to Nagano discloses an invention making use of the reduction in the engaging tooth height of the deformed tooth or the affixed tooth. The fixed-point gear changing is attained by means of the relay engagement of the intermediate teeth. However, this design is costly, and the split tooth also weakens tooth strength and has negative impact on safety.
U.S. Pat. No. 5,413,534 entitled “Chain Shift Aiding Structure for Bicycle Sprocket” to Nagano discloses a gear changing path which is provided with a support projection for bracing the waist of the sprocket chain at the time when the gear changing takes place, thereby averting the chain shifting failure which is brought about by the fall of the sprocket chain before the chain has arrived at the engaging teeth.
U.S. Pat. No. 5,085,621 entitled “Multi-stage Sprocket Gear for Bicycle” to Nagano discloses a sprocket having the design of the slanted pocket of recess for averting the sprocket changing outer chain plate and for bracing the waist portion of the sprocket chain, so as to enhance the chance of success of gear changing.
The prior art also teaches the use of transmissions including a “variable diameter sprocket.”For example, U.S. Pat. No. 4,634,406 entitled “Multiple Speed Transmission for Bicycles” to Hufschmid, discloses a transmission, in which one of the front and rear groups of sprockets is replaced with a segmented sprocket. The segmented sprocket has a number of radially movable segments that engage the chain. By moving the segments inwardly or outwardly, the effective diameter of the segmented sprocket can be changed, thereby varying the gear ratio of the transmission.
Unfortunately, this transmission has not been commercially acceptable, because it is complicated, too heavy, inefficient, and is not well suited to automation with low powered actuators.
In addition, the prior art teaches various types of gear-tooth configurations and placements to facilitate derailing of the chain from one sprocket to an adjacent sprocket. As can be seen in such patents, tooth configuration has heretofore been optimized to allow the chain to twist axially sideways so as to more easily be derailed from one sprocket to an adjacent sprocket. The positions of the teeth on the sprockets are likewise optimized to facilitate derailing. For example, a “missing” tooth configuration has been known to facilitate derailing as the “missing” tooth space more easily allows the chain to be grasped by the tooth of an adjacent sprocket without slippage. U.S. Pat. No. 4,348,200 entitled “Multi-speed Sprocket Gear for a Bicycle” to Terada discloses a “missing” tooth configuration wherein a reduced radius, or recess, is provided at the point of the “missing” tooth. A tooth is positioned in the reduced radius, or recess, to facilitate shifting of the gear of the chain from one sprocket to an adjacent sprocket by means of the tooth positioned in such recess.
U.S. Pat. No. 5,192,249 entitled “Multi-step Bicycle Transmission Sprocket Gear” to Mu discloses multi-step transmission sprocket gear and includes a large sprocket gear, an intermediate sprocket gear, and a small sprocket gear. The intermediate sprocket gear is provided in the top surface of a predetermined tooth with a first recess and is further provided with a chain-guiding recess located in the back side of a plurality of adjoining teeth along the reverse revolving direction of the predetermined tooth. The large sprocket gear is provided in the top surface of a predetermined number of teeth thereof with a second recess corresponding in location to the chain-guiding recess of the intermediate sprocket gear in the reverse revolving direction. The sprocket chain is capable of shifting from the small sprocket gear to the intermediate sprocket gear with speed, thanks to the first recess and the chain-guiding recess of the intermediate sprocket gear. The second recess of the large sprocket gear serves to avert the deflection of the sprocket chain so as to eliminate the mechanical interference of the large sprocket gear by the sprocket chain when a transmission process is in progress.
A disadvantage presented by the MU patent is that the chain is not always engaged to the teeth, the recesses do not overlap, thus a slip of the chain can occur.
Finally, U.S. Pat. No. 6,293,884 entitled “Cluster Sprocket for Bicycle Transmissions and Other Prime Movers” to the present inventor, the disclosure of which is hereby incorporated by reference, discloses a multi-sprocket gear including a plurality of progressively-sized sprockets that are positioned adjacent to one another to define an increasing diameter sprocket gear.
A disadvantage presented by the Chattin patent is that the cluster system does not provide a smooth derailing of the chain from one sprocket to an adjacent sprocket.
While the foregoing sprocket gears noted above have been recognized in the industry as providing some solutions to the problem of slippage during derailing, there is nevertheless a substantial need in the derailer industry for cluster sprockets having configurations that optimize the derailing of the chain from one sprocket to an adjacent sprocket.
What is needed is a reasonably simple variable ratio cluster sprocket—which is reasonably light in weight, yet reasonably robust, reasonably inexpensive to fabricate and readily shifted from one ratio to another.